In the US, once you get to 18,000 ft, you set your altimeter to 29.92. My question is why? Unless the pressure outside is 29.92 your true altitude and your altimeter will not match up. The pressure doesn't change just because you reach 18,000 ft. So why can't you leave your altimeter at the real pressure setting the entire flight?

"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"

What pressure reading would you use? The one from the airport you took off from? How about the guy coming the other way, with a different pressure reading from the airport 1,500 miles away which departed from? To provide positive altitude control, everyone uses the same setting, so you don't have aircraft that should be at different altitudes running into one another.

What moose said, and in addition, at FL180 and above (essentially class A airspace to FL600), the airspace is occupied mostly by high speed enroute jet aircraft. Imagine how many times a pilot would theoretically have to reset his/her altimeter on a coast-coast flight, especially crossing multiple pressure systems (mid latitude cyclones).

Like others said, at such altitudes and speeds, actual altitude doesn't matter; its that everyone is using the same scale and not bumping into each other is what matters.

It's so everyone has a common barometer to base their seperation by. Does it matter what the real altimeter reading is at your original airport or destination airport? ATC can't provide positive seperation, much less of 1,000' or less, if everyones using a different 'standard'. You set it to 29.92 and everyone is 'on the same page'. Its like Zulu time, your local figures dont matter you have to have a common denominator to provide safety. I can't imagine the chaos if everyone was using their own localy correct barometric pressure. Its standardized for safety.

Quoting Northwest727 (Reply 4): Imagine how many times a pilot would theoretically have to reset his/her altimeter on a coast-coast flight, especially crossing multiple pressure systems (mid latitude cyclones).

One can't forget the associated chaos with ATC constantly reading altimeter settings to aircraft in the vicinity. I hear it enough on an IFR flight plan under FL180, traveling a measly 130kts...can't imagine the headache if I were traveling at jet speeds.

Another thing to consider is that small missettings create larger and larger errors in altitude as altitude decreases. At sea level a 1 inch underset (admittedly rather a large error) on your altimeter results in about a 940 ft (assuming something close to a standard day). At 20,000ft it's 1800ft, and at 40,000ft, the error is some 4100ft.

So even small differences in altimeter settings at high altitude can cause aircraft to miss their assigned altitude significantly, making normal separation impossible. At low altitudes, there's enough slack.

Quoting ShyFlyer (Reply 6):To further expand, 29.92 is used as it is the pressure used in the International Standard Atmosphere model.

No it is not, it is 1013.25 HPa. 1013 HPa is the standard sea level pressure used internationally in aviation, only a few countries used the non-standard 29.92, they are also normally the ones that use FAA TERPS instead of ICAO PAN-OPS.

Like the old joke

ATC: Pan Am 1, descend to 3,000 ft on QNH 1019.

Pan AM 1: Could you give that to me in inches?

ATC: Pan Am 1, descend to 36,000 inches on QNH 1019

We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar

Quoting moose135 (Reply 2):What pressure reading would you use? The one from the airport you took off from? How about the guy coming the other way, with a different pressure reading from the airport 1,500 miles away which departed from? To provide positive altitude control, everyone uses the same setting, so you don't have aircraft that should be at different altitudes running into one another.

You take off using the pressure at your departure airport.
After airborne... if you enter an area where the "area QNH" is different or your current area has a change.... ATC will tell you to change to new setting.
Once you go above the transition altitude (11000ft where I am... 18000 in US... and whatever it is in the regulation valid for whichever area you're in) you go to 1013mb or 29.92 (US)... the QNE...
You stay at the QNE until you descend below the transition level (FL130 where I am... FL180 in the US)... the ATC will tell you the area QNH that's valid at the time... or you might get given the QNH for your destination airport immediately... and use it once you go below the transition level.

If your flight never go above the transition altitude... then you get given whatever QNH is valid for your area... new QNH when there's a change or when you enter a new area... until your destination.

That way... for low levels... everyone at the same area will use the same QNH.

Which translates to 29.92. Not exactly, but since we aren't calculating performance values (which would use 1013.25 hpa and +15*C) it will suffice for this discussion, especially since the OP is in the US and asking about a US procedure. You are the most technically correct, though, and I appreciate the correction/additional information.

From my first link:

Quote:The most commonly used atmosphere model is the International Civil Aviation Organization (ICAO) Standard Atmosphere, of which the main part is the International Standard Atmosphere (ISA). At all altitudes of interest to an aircraft performance engineer, the ISA model is the same as that of the 1976 U.S. Standard Atmosphere, and the MIL-STD Standard Atmosphere.